This invention relates to the preparation of a resistance memory using metal oxide thin films, and specifically to a form of non-volatile memory device which incorporates an electric-pulse induced reversible resistance change as a memory element in a non-volatile cross point electrically programmable RAM array.
The Pr0.7Ca0.3MnO3 (PCMO) metal oxide thin films demonstrate reversible resistance change via applying electric pulse, were grown on both epitaxial YBa2Cu3O7 (YBCO) and partial epitaxial Pt substrates via pulsed laser ablation (PLA) technique, as described by Liu et al., Electric-pulse-induced reversible resistance change effect in magnetoresistive films, Applied Physics Letters, 76, 2749, 2000; and Liu et al., U.S. Pat. No. 6,204,139, granted Mar. 20, 2001, for Method of switching the properties of perovskite materials used in thin film resistors. X-Ray diffraction (XRD) polar figures confirm the epitaxial properties of PCMO thin films.
U.S. Pat. No. 6,204,139 describes the resistance change which occurred when electric pulses are applied at room temperature to PCMO thin films. The PCMO thin films were deposited on both epitaxial YBCO and partial epitaxial Pt substrates by pulsed laser deposition (PLD). In the ""139 reference, the polarity of the electric pulse determined the character of the resistance change, i.e., increase or decrease.
An electrically programmable resistance, non-volatile memory device, operable at room temperature, was made of PCMO epitaxially grown on YBCO on LaAlO3, as published by Liu et al. This type of memory may be reversibly programmed by a reversed short electrical pulse, or, as described in the above-identified related Application, by application of electrical pulses of varying length, regardless of polarity. The memory cell is able to produce either single bit or multi-bit information. However, the PCMO must be in crystalline form, which requires that the PCMO must be grown on a specific bottom electrode, such as YBCO, which is not compatible to the state-of-the-art silicon integrated circuit technology. The growth, or crystallization, temperature is relatively high, e.g.,  greater than 700xc2x0 C., which makes integration of the device into state-of-the-art integrated circuit very complex. In addition, it is not possible to cover the full circuit area with a single grain of PCMO. As the properties of a memory cell which is fabricated on a single grain PCMO crystal and the properties of a memory cell which is fabricated on a multi-grain PCMO crystal, which covers the grain boundary area, are not the same, circuit yield and memory performance problems will occur.
A method of forming a multi-layered, spin-coated perovskite thin film on a wafer includes preparing a precursor solution comprising: mixing solid precursor material into acetic acid forming a mixed solution; heating the mixed solution in air for between about one hour to six hours; filtering the solution when cooled; placing a wafer in a spin-coating mechanism; spinning the wafer at a speed of between about 500 rpm to 3500 rpm; injecting the precursor solution onto the wafer surface; baking the coated wafer at a temperature of between about 100xc2x0 C. to 300xc2x0 C.; annealing the coated wafer at a temperature of between about 400xc2x0 C. to 650xc2x0 C. in an oxygen atmosphere for between about two minutes to ten minutes; repeating the spinning, injecting, baking and annealing steps until a perovskite thin film of desired thickness is obtained; depositing a top electrode on the thin film; and annealing the perovskite thin film at a temperature of between about 500xc2x0 C. to 750xc2x0 C. in an oxygen atmosphere for between about ten minutes to two hours.
It is an object of the invention to provide a multi-layered, spin-coated PCMO thin film for use in non-volatile memory devices.
Another object of the invention is to provide a precursor solution suitable for PCMO spin-coating.